(1) Field of the Invention
This invention relates to improvements in the security of locks, for example, but not limited to mortise locks. In particular, the invention relates to a novel deadbolt for use in locks, the novel deadbolt having improved resistance to attack by persons attempting unauthorised access to a secure area.
(2) Description of the Art
It is not uncommon for persons to gain unauthorised access to a domestic or business property by destroying a lock on the external door to the property. Typically a lock will comprise a bolt member associated with a keyhole mechanism which, when the lock is closed, fits into a similarly sized slot. The bolt member may then be released from the slot when an appropriate key is fitted in the keyhole and turned in an appropriate manner. One means by which unscrupulous persons gain unauthorised access is by machining into soft materials surrounding the slot and destroying the bolt. For example, where the lock is a mortise lock securing an external door to a domestic property, an unscrupulous person may drill through the door frame into which the dead bolt of the lock fits and penetrate the metal (typically brass) of the deadbolt. The deadbolt is destroyed and the door may be freely opened.
At present, in the UK, insurance premiums for domestic properties may be reduced where the external doors of the property are secured by locks which conform to British Standard BS3621. This standard is expected to be superseded by a European Standard prEN12209-½ which recommends that locks for domestic use be drill resistant. Thus there is a need for a drill resistant lock.
The current trend in European lock manufacture has been to move to changing the material for the entire lock, replacing traditional soft metals such as brass with harder metals such as martensitic steels. Whilst these harder metals are inherently more resilient to attack by machining tools such as drills, their very different physical nature requires the use of different tools and methods of manufacture. It will be understood that retooling can be a costly exercise to industry. Furthermore, present methods of manufacture rely on post process machining to correct small dimensional variations between lock mechanisms and dead bolts in a batch. The use of hard steels in place of softer metals such as brass would likely require high precision casting methods which may be prohibitively expensive. It is also of note that many of these hard steels are prone to corrosion when subjected to prolonged exposure to damp atmospheric conditions. Corrosion resistance may be improved by chemical treatment of the steels but this again raises the overall manufacturing costs.
It is known to provide tamper resistant locks by including in a bolt a core of relatively hard material. For example, FR 2740498, U.S. Pat. No. 5,678,432, U.S. Pat. No. 4,446,707 and U.S. Pat. No. 3,799,592 all describe arrangements where a core of hardened steel is incorporated in a bolt. Such arrangements are described as “saw resistant”. Whilst such arrangements may indeed provide resistance to sawing, it is to be understood that hardened steel is not as hard as tungsten carbide commonly used for the manufacture of drill bits and is accordingly not resistant to penetration by drilling.
GB 1338720 discloses a door lock having an insert of solid, hard ceramic. In that arrangement, the ceramic plate is, necessarily contained on all sides by a metallic material so that particles of the ceramic may be operable to act on the drill bit, even after the ceramic insert has been shattered on initial impact with the bit. Such an arrangement would not work against a drilling operation if the insert were at a surface of the bolt as the ceramic would shatter and fall out of the bolt providing access to the underlying, machinable metal body.
GB 1392268 discloses a padlock having a shackle containing a core of hardened carbide particles in a binder of solder. The carbide particles are closely packed at high density. The shackle is effective against sawing or machining operations in a similar manner to the bolt as described in GB 1338720 and suffers from similar disadvantages. The core is again, necessarily confined on all sides by the metal body of the shackle.